1. Field of the Invention
Embodiments of the invention are generally directed to the field of semiconductors. More particularly, embodiments of the invention are directed to semiconductor devices having improved thermal characteristics and methods associated therewith. Most particularly, embodiments of the invention are directed (but not limited) to gallium nitride (GaN)-based monolithic microwave integrated circuits (MMICs) exhibiting improved heat dissipation and to methods for obtaining improved thermal performance from such devices.
2. Description of Related Art
MMICs are semiconductor devices that are increasingly being used in radar, communications, antenna and sensor applications, as well as others, which demand small size and high performance. Various semiconductor technologies have been developed over the past 30 years. Persons skilled in the art will recognize these technologies to include Field Effect Transistor (FET), high electron mobility transistor (HEMT) including pseudomorphic high electron mobility transistor (PHEMT) and metamorphic high electron mobility transistor (MHEMT), vertical PIN (VPIN) diode, and heterojunction bipolar transistor (HBT). These technologies have traditionally been based on the material properties of gallium arsenide (GaAs), silicon (Si), silicon carbide (SiC), silicon germanium (SiGe), and indium phosphide (InP), for example.
New semiconductor materials and semiconductor/substrate combinations continue to be sought for higher frequency operation, greater power density handling capability, higher operating voltage/lower current operation, improved operational efficiency, operation under more demanding operating conditions (e.g., heat load), and other reasons. Currently, gallium nitride (GaN) and aluminum-gallium nitride (AlGaN)-based semiconductors are receiving considerable attention. They have demonstrated greater power handling capacity and improved, measured performance parameters over the more traditional technologies referred to above. For a general discussion on MMICs and more focused disclosure on GaN and AlGaN-based MMICs, the interested reader is referred to the following resources, the disclosures of which are hereby incorporated by reference in their entireties to the fullest extent allowed by applicable laws and rules: Kayali et al., GaAs MMIC Reliability Assurance Guideline for Space Applications (Dec. 15, 1996) at http://parts.jpl.nasa.gov/mmic/contents.htm; M. Germain, IMEC improves GaN HEMTs with ceramic substrates, (October 2005) at http://www.compoundsemiconductor.net/articles/magazine/11/10/2/1; MMIC semiconductor tradeoffs at http://www.microwaves101.com/encyclopedia/MMICsemi.cfm, Alekseev et al., Broadband AlGaN/GaN HEMT MMIC Attenuators with High Dynamic Range, 30th European Microwave Conference (GMS) (2000) at http://www.eecs.umich.edu/dp-group/GAN/emw2000.pdf, Sanabria et al., A GaN Differential Oscillator With Improved Harmonic Performance, IEEE Microwave And Wireless Components Letters, 15, 7, pp 463-465 (July 2005).
Reports in the literature predict that by 2007, a typical microprocessor of about 1 cm2 will contain over one billion transistors. The higher operating frequency and power handling capability of current and prospective semiconductor devices, e.g., GaN-based MMICs, combined with an ever increasing packaging density, minimization constraints and reliability demands are driving the need for more efficient thermal management. A discussion of challenges and solutions pertaining to thermal issues of semiconductor devices is disclosed in Wilson, Thermal Issues in GaAs Analog RF Devices at http://www.electronics-cooling.com/html/2002_february_a1.html. An overview of semiconductor cooling concepts and implementation is presented in Ohadi, Thermal Management of Next Generation Low Volume Complex Electronics (May 13, 2003) at http://www.vita.com/cool/pres/0845-Ohadi.pdf. The disclosures of both of these references are hereby incorporated by reference in their entireties.
One exemplary thermal management solution includes the chemical vapor deposition of a thin diamond layer on the semiconductor substrate to increase thermal conductivity. Another known solution involves the construction of air bridges over gate and drain terminals, providing double sided cooling and thermal paths to separate heat sink locations. These approaches have met with varying degrees of success. For example, the air bridges are thermally far removed from the heat dissipating area of the die, which limits the effect of additional topside heat sinking. In any event, current approaches do not yet offer optimum thermal management, while the demands increase literally daily.
In view of the foregoing, the inventor has recognized a need for improvement in the thermal management of semiconductor devices, particularly GaN-based MMICs (but not excluding others). The improvements offered by the embodiments of the invention will contribute to advancing the packaging, performance, reliability, application, cost and other principal considerations of new generation semiconductor materials, devices and processes.